Add new sample simple_sdf (#2835)

Add a new sample, `simple_sdf`. This is similar to `simple_shader`, but
it focuses on using Signed Distance Functions within the fragment
shader.

Relates to issue
[#183043](https://github.com/flutter/flutter/issues/183043)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [ ] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).


If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md
[changelog]: ../CHANGELOG.md

README.md

@@ -28,6 +28,7 @@ Googler's, you can freely add samples to the [flutter/demos] repository.
 * [`navigation_and_routing`] - A sample that shows how to use [go_router] API to handle common navigation scenarios.
 * [`pedometer`] - A demo of a plugin that leverages FFIgen & JNIgen to call platform APIs directly from Dart code.
 * [`platform_design`] - This sample project shows a Flutter app that maximizes application code reuse while adhering to different design patterns on Android and iOS.
+* [`simple_sdf`] - A simple [Flutter fragment shaders] sample project showing how to draw Signed Distance Functions with the FragmentShader API.
 * [`simple_shader`] - A simple [Flutter fragment shaders] sample project.
 * [`testing_app`] - A sample app that shows different types of testing in Flutter.
 * [`web_embedding`] - This directory contains examples of how to embed Flutter in web apps (without iframes).
@@ -123,6 +124,7 @@ If you run into a bug in one of the samples, please file an issue in the
 [`navigation_and_routing`]: ./navigation_and_routing
 [`pedometer`]: ./pedometer
 [`platform_design`]: ./platform_design
+[`simple_sdf`]: ./simple_sdf
 [`simple_shader`]: ./simple_shader
 [`testing_app`]: ./testing_app
 [`web_embedding`]: ./web_embedding